board: MCR3000: cleanup config

Some config is redundant with Kconfig. Fix it.
Also remove unused configs
Move SDRAM_MAX_SIZE in the only place it is used

include/environment.h already defines CONFIG_ENV_SIZE
from CONFIG_ENV_SECT_SIZE and defines CONFIG_ENV_ADDR as
(CONFIG_SYS_FLASH_BASE + CONFIG_ENV_OFFSET)

remove BOOTARGS as bootargs is set by the different boot commands

Fix CONFIG_SYS_INIT_RAM_ADDR and CONFIG_SYS_INIT_RAM_SIZE to be in
line with CPM DPRAM organisation

Remove CONFIG_SYS_GBL_DATA_SIZE, CONFIG_SYS_GBL_DATA_OFFSET and
CONFIG_SYS_INIT_SP_OFFSET which are unused

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>